1. Field of the Invention:
The invention resides in the field of heat transfer devices and more particularly relates to recuperators for recovering waste heat from flue gases emanating from flue gas stacks.
2. Description of the Prior Art:
Recuperators as known in the prior art may have many different shapes and dimensions. The radiation type recuperator most suitable for high temperature heat recovery is essentially a vertical concentric double cylinder, forming a heat exchanger in which flue gas passes through the inner shell vertically upward and combustion air to be preheated is passed through the space between the two cylinders. A down flow arrangements is possible as well, but since most installations call for a discharge of the flue gas through a stack into the atmosphere, an upward flow is most common. Since the recuperator can act as a stack, this type of recuperator is often referred to as a stack type recuperator. An example of such apparatus is shown in U.S. Pat. No. 3,346,042, issued to the applicant.
A radiation recuperator of this type consists basically of two concentric large diameter metal shells welded together at each end by way of air inlet and outlet headers. Flue gases from a furnace pass through the inner shell while combustion air passes through the narrow gap between the shells. Heat from the flue gas is transmitted to the inner shell or heating surface mainly by gas radiation which may account for as high as 75 to 95% of the total heat transferred; additional heat is transferred by convection due to the flow of the flue gas through the recuperator, as well as by radiation from the hot flue canal into the recuperator. On the other side of the heating surface of the recuperator, the combustion air moves with high velocity, picking up heat by convection from the inner shell. Because the inner shell is much hotter than the outer shell, heat is radiated also to the outer shell across the air gap, resulting in a secondary heating surface formed by the outer shell from which the combustion air picks up heat by convection as well. Overall, a very complex system of heat transfer takes place between the flue gas and combustion air. Despite extremely high flue gas temperatures of up to 2500.degree. F. entering such a recuperator, and air preheats of up to 1400.degree. F., actual metal temperatures may not be higher than 1600.degree. F. under normal operating conditions. If, however, the fuel input into the furnace is turned down, for example to 25% or less of maximum conditions, metal temperatures may rise to 1800.degree. F. In case of power failures, temperatures may briefly reach 2000.degree. F. and more. As stated above, the greatest part of the heat contained in the flue gas is transmitted to the heating surface or or inner shell by gas radiation, which does not depend on the velocity of the flue gas, while the cooling of the recuperator from the combustion air side depends entirely on velocity. The result is higher metal temperatures of the recuperator under low flow and power failure conditions, assuming that the flue gas temperature entering the recuperator is maintained at a high level, which is often the case at low fire conditions.
The above described high temperature conditions require these recuperators to be constructed of large, carefully made, metallic cylinders highly resistant to oxidation, corrosion and temperature. Despite refined design techniques, these prior art devices eventually deteriorate due to the environmental stresses they are constantly subjected to.
The present invention is intended to replace or augment radiation recuperators of the large cylindrical shell type while avoiding the difficulties enumerated above, since gas radiation is a function of approximately the fourth power of the absolute gas temperature and therefore is highly dependent upon flue gas temperatures.